The present invention is directed to a system for maintaining control over the state of charge of a battery. It is especially useful for monitoring rechargeable storage batteries such as those used in battery powered vehicles. These vehicles may include various battery powered tools, such as fork lifts or the like, and the invention will be described in detail in the context of such a vehicle. However, the inventive system may also be used in any battery powered system which employs rechargeable or nonrechargeable batteries. The state of charge of the battery is detected by monitoring transient reductions in terminal voltage during operation of the vehicle and using this information and the relationship between battery capacity and the magnitude and duration of fluctuations in battery terminal voltage to derive state of charge information. A number of systems monitoring battery depletion in this manner are disclosed in U.S. application Ser. No. 538,463, now abandoned, entitled "System for Measuring Battery Depletion by Monitoring Voltage", the disclosure of which is hereby incorporated by reference.
The depletion of battery charge is monitored by generating and integrating a signal related to the magnitude and duration of fluctuations in the battery terminal voltage. The state of charge of the battery may be displayed by any device such as an electric meter that is calibrated in terms of percentage charge remaining in the battery. The display is similar to a display showing the fuel remaining in a conventional gasoline powered vehicle and is therefore quite easy for an operator familiar only with gasoline powered vehicles to read and understand. The system is also provided with means for detecting when the remaining charge in the battery has been depleted below a predetermined level and disabling the various tools on the vehicle, leaving only those systems that are essential for the return of the vehicle to a battery charging station.